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Abstract

The drift issue induces slow drifting of the optimum operating point
for high efficiency or large nonlinearities in analog optical links, and requires
complex control of the offset bias voltage for achieving high extinction ratio
in digital optical links. We discuss and analyze the different sources of
the drift in commercially LiNbO$_3$ Mach–Zehnder modulators. The different extrinsic and intrinsic
origins are compared in terms of phase shift and the different corresponding
orders of magnitude are given, pointing out the predominant role of the intrinsic
(dc) drift. We show the large role played by the electrical inhomogeneities
at the surface of the LiNbO$_3$ substrate by highlighting the link between the time dependence
of the dc drift and the electrical conductivity measured at the surface and
in the volume of the LiNbO$_3$ substrate. This allows to propose a solution to the drift issue
which consists in the engineering of the electrical conductivity of the lithium
niobate substrate.

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